1. Field of the Invention
The invention relates to an electric power steering apparatus which is provided with an electric motor and which assists the turning operation of a driver's steering wheel performed by a driver. The invention relates also to a method for controlling such electric power steering apparatus.
2. Description of the Related Art
An electric power steering apparatus for a vehicle, which detects the steering torque applied to a steering wheel that is turned by a driver (hereinafter, referred to as a “driver's steering wheel”), and which causes an electric motor to produce the steering assist torque based on the detected steering torque has been used. A controller controls the steering assist torque. The controller calculates the target assist torque based on, for example, the steering torque detected by a steering torque sensor and the vehicle speed detected by a vehicle speed sensor, and controls the amount of electricity passed to the electric motor based on the target assist torque to produce a desired steering assist torque.
With such electric power steering apparatus, if a malfunction occurs in, for example, a sensor, an appropriate steering assist torque is not produced. Therefore, in an electric power steering apparatus described in Japanese Patent Application Publication No. JP-2004-210065 (JP-A-2004-210065), when a malfunction signal from a steering torque sensor is detected, the amount of electricity passed to an electric motor is gradually reduced to stop the assistance to the turning operation of a driver's steering wheel performed by a driver (hereinafter, referred to as the “steering assist”).
However, with the electric power steering apparatus described in JP-A-2004-210065, whenever the steering torque sensor malfunctions, the steering assist is stopped. After the steering assist is stopped, the driver is required to produce a greater force to turn the driver's steering wheel, which increases the burden on the driver. If a malfunction caused in the sensor is not a major one, the driver may just feel a slight sense of discomfort in the steering operation. For example, the driver may just feel pulsatile torque fluctuations when turning the driver's steering wheel, or inconsistent steering feel between clockwise and counterclockwise turns of the driver's steering wheel. In such a case, it is not always necessary to stop the steering assist.